Oxidation of organic sulfides



United States OXIDATION OF GRGANIC SULFEDES 2 Claims. (Cl. 260-607) Thisinvention relates toa method for the'manufacture of sulfoxide, and moreparticularly, to an improved process for the manufacture of lowmolecular weight sulfoxides from the Corresponding sulfides.

United Statesletters Patent No. 2,581,050 relates to the vapor phaseoxidation of dimethyl sulfide to dimethyl sulfoxide using anoxygen-containing gas which also includes a small amount of anoxygen-transmitting nitrogen oxide (such as N02), using reactiontemperatures sufficient to maintain the dimethyl sulfide in the vaporphase.

United States Letters Patent No. 2,702,824 relates to the liquid phaseoxidation of low molecular dialkyl sulfides to the correspondingsulfoxides by bubbling a gaseousmixture through the liquid dialkylsulfide at a reduced temperature to maintain the dialkyl sulfide in theliquid phase. The gaseous mixture bubbled through the liquid dialkylsulfide comprises an oxygen-containing gas and a small amount of anoxygen-transmitting nitrogen oxide;

In eachof the foregoing operations, air is preferred as theoxygen-containing gas for obvious economic reasons; In carrying outthese operations using air, however, prior to the instant invention itwas found that the operating conditions were difficult to control andthe yields obtained left something to be desired. The instant inventionis based in part on the discovery that the use of pre-dried atmosphericair in the foregoing operations results in a distinct improvement inthe, yields andta distinct improvement in the control of the operatingconditions. Compared to the quantities of other ingredients used theamount of moisture in atmospheric air is extremely small. Nevertheless,it hasbeen found that this moisture in the air apparently has a verydis: tinct. harmful effect upon the reaction, and the instant inventioncontemplates the removal thereof from the air before carrying out thereaction. The instant invention. also contemplates the use ofsubstantially anhydrous sulfides in thereaction mixture, since it. hasalso been, discoveredthat the relatively small amount of moistureordinarily present in the sulfides has a definitely harmful. effectuponthe reaction.

It is, therefore, an important object of the instant invention toprovide an improved method for oxidizing low molecular weight sulfides,to the corresponding sulfoxides.

It is a further object of the instant invention to provide an improvedmethod for the manufacture of low molecular weight sulfoxides from thecorresponding sulfides by an oxidation process which is madeindustrially superior by removal of moisture from the reactants prior tocarrying out the oxidation reaction.

Other and further objects, features and advantages of the presentinvention will become apparent to those skilled in the art from thefollowing detailed disclosure thereof.

The instant invention consists in a process for the manufacture of a lowmolecular weight sulfoxidefrom thecorresponding sulfide by oxidationthereof, which comprises drying atmospheric air to reduce the relativeatent C 2. humidity thereof by at least one-half and then contacting thedried air with the sulfide in the presence of an amount of oxygentransmitting. nitrogen oxide at least suflicient to catalyze theoxidation.

The sulfides which may be used in the practice of the.

instant invention include the low molecular weight dialkyl sulfideshaving the following formula:

S isa nuclear atom. Such compounds include 5 to 6' membered'ringshaving 1. to 2 nuclear S atoms and the remainder C atoms, in saturatedor unsaturated rings, with l to, 2. substituents on the C atoms,preferably C C.,.alkyl substituent-s'. Typical types of compounds arebased on the thiophene and tetramethylene sulfide nuclei:

wherein each is H or a C -C alkyl group but not more than 2 X sarealkyl" groups and the remainder are Hs. Examples includetetramethylene sulfide, l-methyl tetramethylene sulfide, Z-methyltetramethylene sulfide, 1,3-dimethy1' tetramethylene sulfide, etc. up tol-butyl tetramethylene sulfide.

Another heterocyclic compound is based on a 6 membered ring nucleusrwherein Y' may, be S or CX and: the, Xs have the, meaning hereinbeforegiven. Examples. include pentamethylene sulfide, l-methyl pentamethylenesulfide, 2- methyl pentamethylene sulfide, 3-methyl pentamethylenesulfide, 1,2-dimethyl pentamet-hylene, sulfide, etc. up to the butylpentamethylene sulfides; and dithiane. (i. e. diethylene disulfide),l-methyl dithiane, etc. up to the butyl dithianes. Members of thesegroups whichare nor.- mally solids can be dissolved in, for example,dimethyl sulfoxide to provide a liquid composition of the sulfide thatis oxidized in the'process of the invention.

In. the practice of the invention, the organic sulfide may be oxidizedvin either the liquid or the vapor phase, but in the case of the higherboilingsulfides it is ordinarily preferable to employ liquid phaseoxidation at lower temperatures rather than athigher temperaturesrequired to volatilize-the sulfide. Ingeneral, sulfides boiling attemperatures above about C. should be oxidized in the liquid phaserather than the vapor phase; and sulfides which are solids attemperatures above about 75 C. should be oxidized in the liquid phase insolution in a sulfoxide such as dimethyl sulfoxide (or anyother solventwhich is not reactive with the ingredients here present, namely, thesulfide, sulfoxide, NO and N0 preferably, the. aforementionedtemperature of 75 C. should be about 40 (3., since this is a preferredmaximum temperature for carrying out the instant oxidizing reaction. Theminimum temperature is, of course, around 10 C, or the minimumtemperature at which Patented, Mar. 4, 1-958 appreciable oxidationtalces 'place. The preferred reac- V tion temperature range is about2540'C.

The sulfides used in the practice of the instant invendried air, stillbetter results are'obtained using th'e substantially anhydrous sulfide.

same in liquid or vapor form over a suitable drying agent, such ascalcium oxide.

Although air at 80 F. having 50% relative humldity contains only about0.01 pound of water per pound of air this 'apparentlyinsignificantamount of water has been V foundito beextremel'y. harmful. In fact,itis.possible to The sulfide 'rnay beobtained in substantially anhydrousform -(i'. e., containing less than about 0.1% by weight of water)bypassing the obtain adistinctl advantage in the practice of the instantinvention 'merelybyreducing the relative humidity. of the air byatleastone-half' of the initial relativehumidity. Preferably, however,this reduction in relative humidity also results in a reduction of themoisture content in the air to belowabout 0.002 pound per pound of air.Best resultsare obtained if the moisture contentis reduced to belowabout 0.001 pound per pound of air.

The humidity in the air may, of course, be reduced by passingthe airthroughlany sort of drier or dehumidifier. A preferred process .involvespassing the atmospheric air over' calcium oxide, which efiectsareduction in the moisture content to a relative humidityof approximately1% at 100 F. (which is the moisture content ofapproximately 0.0005 poundper pound of 'bone dry air). ,The dehumidification of air is relativelyeasy and economic '4 reaction. This causes difiiculties in control ofthe re action. In addition, the constant consumption of. the I catalystrequires the use of greater amounts of the nitrogen oxide than wouldordinarily be required; Also, the acids formed must subsequently beneutralized in the sulfoxide product. It is further believed that thepresence of the moisture actually interferes with the catalytic functionso that lower yields of the sulfoxide are obtained. V V As ademonstration of the instant invention, dimethyl? sulfide is evaporatedat'a rate of 0.7 gram per minute i nto an air current of 680 millilitersper minutecontaining nitrogen dioxide introduced into the air currentat-a rate of 45 milliliters per minute. 7 The resulting gaseous mixtureis passed into the first of a series of four reaction. chambers, eachconsisting of a glass tube 4.2 centimeters in diameter and 100centimeters in length. The reaction starts immediately,-the temperatureof the reaction ture reaches about 55 C.1in the first reaction tube",about 45 C. in the second tube, abut 36 C; in the third tube and aboutC.'in the last tube. The crude'product."

andfor this reason yery definite advantage from a prac- V tical point ofview is obtained merely'by drying the air, while using the commercial(undried) sulfide. In the vapor phase reaction, however, the rier forthe sulfide to conduct. the same into the reaction zone, andd'ehumidification of the air-sulfide vapor 'mix-.

ture can be carn'edont to advantage prior to reaction.

. The amounts and types of oxygen-transmitting nitrogen oxides used inthe practice of the instant invention are described in detail in saidU..S. Patents No. 2,581,050

and 2,702,824 and neednot be described with'further particularityherein." Such gases include NO, N0 N 0 N 0 etc., but the predominatingcatalytic func- 1 tion is generally thought of on the basis of N0 asindicated in the following equations representing the reaction which isbelieved to take place:

oxide grouping. It will be noted that theactual oxidation of the sulfideto the sulfoxide is believed to be causedby reaction between thesulfide. S atom and N0 but the resulting NO is instantly regenerated toNO by the.

presence of the oxygen in the air. ciated, only a small amountof the Aswill be appreoxygen-transmitting nitrogen oxide, such as about 10% to15% by volume,

the oxidation of the sulfide.

Although it is not desired to limit the invention'to any particulartheory,

following equation 3 2NO +H 0 HN0 +HNO Theacids'thus formed tend tocondense out with the sulfoxide product and the concentration ofcatalyst is 4 thus constantly being varied (in fact, reduced) during theair is used as a car-1 contained traces of dimethyl sulfide andnitrogendioxide, about 2% methane sulfonic acid and small amounts (i. e.about 0.3%) of water. In this demonstration, the

air employed is first passed through a tube containing 200 gramsof-calciumoxide so as to reduce the relative humidity of the air toabout 5% at 30 C.; but the dimethyl sulfide employed containedabout'0.4% water. After purification (wherein the dimethyl sulfidewas'removed by heating the product to 150 C., the methane sulfonic acidwas neutralized by adding slaked lime, and ii the dimethyl sulfoxide wasdistille in vacuum) the yield of pure dimethyl sulfoxide was 91% of thetheoretical 7 yield. If the foregoing procedure is repeated using sub--;stantially' anhydrous dimethyl sulfide (containing .less than0.1%moisture) a yield of 94%='is obtained; but if f atmospheric air at about60%. relative humidity -isused with the undried sulfide the yield isonly about 75% (and it is now believed that the reason why ,such smallamounts of water can produce such a notice 7 able efiectupon thereaction is that the water apparentlyi reacts with the N0 to form acidsas indicated in the of commercial grade, the

C. and 35 C. 'in thatorder). Partial' dryingof such' atmospheric air toreduce therelative humidity by about one-half to 30% (at 30 C.)';results in a'- distinct improvementgin the yield to about 78%,;althoughthis is] much less than the yield obtained'by theusefofsubs'tantiallydried air. 1 I I As another demonstratioma single reaction chambercomprising a glass tube 4.3 centimeters in diameter and 50 j centimetersin length is'provided with a gas inlet at the 1 bottom and agas'distributing plate. ;A charge of 500 grams of dimethyl sulfide wasplaced .inwtheg chambei'f which was also filled with filling materialnear the bottom 1' 7 thereof to increase the reaction surface. The upperpart of the reaction chamber is provided with a cooler maiutained atminus 10 C. Air containing;12l5% NO (which'is volume percent based onthe'oxygen content" of i the air) is fed into the bottom of the chamber.The rate at which oxygen is thus fed into the bottom of the chain-' heris 6-10 liters per hour. 7 As the'reaction progressed the temperature.was raised up to 40-45 9 C. and as the reaction was completed thetemperature dropped back to room temperature. Using undried atmosphericair having a relative humidity of about 60% at ,30" C. it was found thatthe yield obtained was about 80% of theoretical. Using air which hasbeen dried to a re1ative"hu-" midityof about 1% at 30 C. (in the mannerhereinbefore; described), the yield obtained is about oftheoretical, 3If substantially anhydrous dimethyl sulfide is used instead" yield isfurther increasedsto about 93% of theoretical. 7 H It will be understoodthat modifications and variations may beeffected without departing fromthe spirit and, scope of the novel concepts of this invention.

I claim as my invention: 1 1; In a process for the manufacture of adi-lower alkyl' sulfoxide from the corresponding sulfide by oxidation ;v

thereof, which comprises contacting the sulfide with an oxidizingatmosphere containing air and an amount of oxygen transmitting nitrogenoxide at least sufiicient to catalyze the oxidation, the improvementwhich comprises contacting the sulfide with air having a moisturecontent below 0.002 pound per pound of air.

2. In a process for the manufacture of a di-lower alkyl sulfoxide fromthe corresponding sulfide by oxidation thereof, which comprisescontacting the sulfide in substantially anhydrous form with an oxidizingatmosphere containing air and an amount of oxygen transmitting nitrogenoxide at least sufiicient to catalyze the oxidation, the improvementwhich comprises contacting the sulfide with air having a moisturecontent below 0.002 pound per 6 pound of air.

References Cited in the file of this patent UNITED STATES PATENTSSmedslund Jan. 1, 1952

1. IN A PROCESS FOR THE MANUFACTURE OF A DI-LOWER ALKYL SULFOXIDE FROMTHE CORRESPONDING SULFIDE BY OXIDATION THEREOF, WHICH COMPRISESCONTACTING THE SULFIDE WITH AN OXIDIZING ATMOSPHERE CONTAINING AIR ANDAN AMOUNT OF OXYGEN TRANSMITTING NITROGEN OXIDE AT LEAST SUFFICIENT TOCATALYZE THE OXIDATION, THE IMPROVEMENT WHICH COMPRISES CONTACTING THESULFIDE WITH AIR HAVING A MOISTURE CONTENT BELOW 0.002 POUND PER POUNDOF AIR.